port "policy-dsp" to 0.5

The policy-dsp.lua script has not been ported to 0.5 yet.

What I think about this script is that it resembles a lot what the monitors/bluez.lua does with the auto-switching loopback source. In the longer term perhaps we can have an architecture in the monitors to create such "device proxy" nodes on the fly with some config. But for now, I don't want to complicate things.

For now, we need to:

• move it under node/ and rename it to something that doesn't have the word "policy" in it (maybe auto-dsp?) - "policy" is an overloaded term that can be confusing (everything WirePlumber does is part of some policy, but what exactly does this script do?!)
• make it retrieve configuration from the main config using Conf.*
• make it use event hooks instead of object managers